In order for a multi-color laser printer to accurately reproduce an image, the laser beam, or beams, for each of the four CMYK colors must be aligned in the scan direction (across the page) and in the process direction (feed direction of the paper). Providing proper alignment of the laser printheads relative to the sheet of media in each direction can be difficult. The optical path taken by the laser beams is often offset in a single polygon mirror optical system. As the laser light passes through these optical systems, light can bend or bow as it moves across its scan. This can generate unwanted scan defects often referred to as artifacts.
Reference is now being made to FIG. 9A which is an illustration of an imaginary grid defined with respect to a surface of a photoconductive drum (or belt) of a laser printer commonly found in the arts. Imaginary grid 902 is defined in relation to photoconductive surface 904 of the drum and is intersected by a plurality of rows 906a-e and columns 908a-e. The intersection of each row and column defines a center of a halftone cell (as shown at location 924 of FIG. 9B).
During latent image formation on the photoconductive surface, each of a plurality of pixels is located relative to the center of one of the halftone cells. If a printing system experiences laser scan process directional errors, such ROS skew and laserbeam bow, the actual location or formation of the pixels on the photoconductive surface deviates from the desired location. This is illustrated in FIG. 9B.
As illustrated in FIG. 9B, a plurality of substantially parallel but bowed dashed lines representing a plurality of scanlines 912, 914, 916, 918 and 920 traced by a laser beam across imaginary grid 902 in scan direction 922, which traverses process direction 910. The ROS skew and laserbeam bow is shown exaggerated to illustrate the positioning problems created by laser scan process errors. Thus, the actual locations associated with a halftone, as depicted by the intersection of the scan lines with columns 908a-e, are subsequently offset in the process direction. This offset becomes more pronounced as the laser beam is scanned from left to right across the photoconductive surface in direction 922. The offset error can occur in an upward direction as opposed to the downward direction shown with respect to scan lines 912-920. In order to minimize the effects of the laser scan errors, the positioning of the laser beam is controlled during the scanning of adjacent pairs of scan lines 914 and 916 to offset the position of pixels of a halftone cell. The redefining of the location of pixels in a halftone cell is accomplished by shifting the pixels in the halftone cell relative to the location of the pixels in an adjacent halftone cell.
Reference is now made to FIG. 10. The imaginary grid 902 is overlaid with a plurality of bowed lines, one of which is shown at 912. Scanline A illustrates uncompensated printing along a beam trajectory without any bowing. Scanline B illustrates uncompensated printing along a bowed beam trajectory. Scanline C illustrates a correction of scanline B by a shifting of data by one to many scanline widths to compensate for beam trajectory bowing. Scanline D represents the result of a halftone cell shifting performed according to the method described in U.S. Pat. No. 7,123,282 to Fields et al. (Oct. 17, 2006).
Reference is now being made to FIG. 11 which is an illustration of a halftone pattern in a non-bowed system corresponding to scanline A of FIG. 10 showing the result of a scan without any beam trajectory bow. Multiple halftone cells 1104 are proximately positioned to render a desired color for halftone 1102. Although the halftone cell is shown having a four-by-four matrix of pixels, the halftone cell may be any dimension of three or greater. The pixels of the halftone cell overlay and are selected to be contained in the halftone cell based on the intensity of the color of that portion of the image. Pixels in a halftone cell are selected based upon studies of the human visual system that interpret the intensity of color.
Reference is now being made to FIG. 12 which is a prior art illustration showing pixels of the halftone cells of FIG. 11. The pixels are shifted in direction 1206 along a boundary proximately located along a border of adjacent halftone cells. At boundary 1202, there is a shifting of a column of pixels (and all subsequent pixels in subsequent halftone cells) that result in the generation of white space 1204. One of the things that is desirable to avoid in the use of halftone imaging is white space. White space leads to a condition that creates a visual artifact that may be perceptible by the human eye. Shifting pixels adjacent to a border of a halftone cell while leaving border pixels of the halftone cell un-shifted generates the visual artifact.
Litho-printers commonly start with halftone angles as follows: Yellow at 0°, followed by a 15° rotation for Cyan, Black at 45°, and Magenta at 75°. Alternately, Cyan can be set at 105° (really 15°+90°) and Magenta at 165° (75°+90°). Rotating these angles by 4-8 degrees is an approach that has been used for both screen printers and flexographers.
Halftone dots are drawn on the diagonal, which many feel hides the digital patterns best. There are speed and efficiency advantages to using angles that are easy to calculate. There are also only a few fixed choices, as you cannot move the pixels around on the printer. Rotating a square 90° has no effect. This helps explain why certain halftone angles are better than others. For example, 30° and 15° are two halftone angles that can be efficiently calculated. Since a quarter turn has no effect on a square grid, rotating a square 120° is the same as rotating it 30°.
In order to determine how far you should rotate the angle set, you must consult with your prepress supplier, service bureau, or RIP software manufacturer, who can tell what angles are supported at the halftone line count you are using. The two most common ones are Yellow 5°, Cyan 20°, Black (K) 50°, and Magenta 80° or Yellow 7.5°, Cyan 22.5°, Black 52.5°, and Magenta 82.5°. Contrasting colors can be printed at any of these angles. When the color separations are printed, the rotated angle positions can be verified with a protractor or angle determiner.
Some halftone screens have an angle of 0°, which means that the cell used to construct a halftone is oriented the same way as the dots on the printer. Halftones with this alignment tend to generate noticeable artifacts since the human eye tends to more easily perceive linearly arranged dots. A higher resolution tends to hide this but so does changing the angle. Since the human eye tends to see the linear arrangement of dots, a higher resolution tends to hide this effect but so does changing the angle. Some consider 45° as the best angle to use since the halftone dots are drawn on the diagonal.
On a single screen, spot registration may not be a big problem. However, when four color separations are combined, minute discrepancies can lead to moiré and color shift. Screen frequency, screen angle, and resolution affect moiré. Bow error affects the screen angle and thus moiré.
Accordingly, what is needed in this art are increasingly sophisticated systems and methods for minimizing visual artifacts generated from laser scan process directional errors in color printing devices, such as printhead skew and laserbeam bow, in a brick-layer halftone structure.